1. Field of the Invention
The present invention relates generally to piezoresistive sensors, and more particularly to piezoresistive sensors for determining airflow conditions on an aircraft wing.
2. Description of the Related Art
The study of airflow characteristics is an important part of basic aeronautical research and design. Air passing over a flight surface, such as a wing, is either laminar, transitional, or turbulent in nature. Air that flows smoothly in a continuous stream is laminar, while an airstream that is rough or broken is turbulent. Transitional airflow, as the name implies, alternates between laminar and turbulent conditions. As the airflow over an aircraft wing becomes turbulent, the fuel efficiency of the aircraft decreases. Therefore, in order to achieve maximum fuel efficiency of an aircraft, it is necessary that the airflow be laminar over as much of the wing surface as possible. Modern aircraft designers are currently experimenting with ways to alter the airflow patterns over an aircraft's wings while in flight in order to increase fuel efficiency. If these methods prove successful, pilots will be provided with controls or automated systems that alter the airflow over the wings to maintain maximum fuel efficiency in any given flight situation.
One way of measuring the characteristics of airflow over a wing is by using a hot-film sensor as disclosed in U.S. Pat. No. 4,727,751. This method involves placing a plurality of hot-film sensors at various locations on the wing and measuring the heat transfer from the sensors to the airstream as they are exposed to laminar, transitional or turbulent airflow. Such hot-film sensors are coupled to an anemometer circuit, which drives them to a constant temperature with a feedback circuit or excites them with a constant current.
Measuring the pressure distribution at the front point of the wing facing the airflow can also be carried out by pressure sensors integrated in the surface. This technique provides the advantage that it does not increase the overall resistance of the aircraft. This type of measuring sensor, however, does pose the danger that measurements may effectively be changed as a result of, for example, ice formation, dirt or bird strike. Consequently, pressure measurements are considerably affected or even impossible, which may lead to total failure of at least individual sensors and even of the air data system.
Pressure load surveys during flight testing require large numbers of pressure measurements distributed across the wing and or body of the plane. Pressure Belts™ have been used for a large number of channels of pressure measurements during a single flight test. Flush ports have also been used where tubing is plumbed from the flush port, a small hole drilled into the body of the plane, to a pressure scanner that may be located a fair distance away. This type of installation is expensive and sometimes requires the use of sacrificial wing parts that significantly raise the cost of the measurements. The tube length needed to connect the port to the scanner causes signal delay and limits dynamic response.
Pressure Belts cannot be used to replace most flush ports. A Pressure Belt cannot be bent around the leading edge or the trailing edge or the tip of a wing. If the Pressure Belt could be bent, it would create perturbation to airflow which corrupts the aerodynamic data.